Sintered bearing

ABSTRACT

Provided is a sintered bearing ( 1 ) including an inner layer ( 2 ) and an outer layer ( 3 ) formed by integral molding, the sintered bearing ( 1 ) having a bearing surface (A) formed on an inner peripheral surface ( 2   a ) of an inner layer ( 2 ). The inner layer ( 2 ) is made of sintered metal containing Fe and a hardness increasing element (such as Ni or Mo). The outer layer ( 3 ) is made of sintered metal containing Fe and no hardness increasing element. A concentration gradient of the hardness increasing element is present at an interface between the inner layer ( 2 ) and the outer layer ( 3 ).

TECHNICAL FIELD

The present invention relates to a sintered bearing.

BACKGROUND ART

A bearing to be used at, for example, a joint section of an arm ofconstruction machinery is required to have excellent wear resistancebecause remarkably large surface pressure is applied to a bearingsurface of the bearing. As the bearing of this type, for example, thereare known a bearing obtained by cutting casting alloy and a bearingobtained by embedding particles of graphite in a sliding surface in aspotted manner. However, both of the bearings have a problem of highproduction cost. Accordingly, instead of those bearings, a sinteredbearing made of sintered metal excellent in moldability is used. Forexample, Patent Literature 1 discloses, as a bearing for constructionmachinery, a sintered bearing that is made of sintered metal obtained bydispersing copper in iron-carbon-based alloy containing martensiticstructure.

CITATION LIST

-   Patent Literature 1: JP 2003-222133 A

SUMMARY OF INVENTION Technical Problems

In order to increase wear resistance, the sintered bearing describedabove is required to have a bearing surface increased in hardness. Inparticular, a shaft and a bearing need to be regularly replaced. In acase where the shaft is more easily replaced than the bearing in termsof a design of the construction machinery, such a design may be employedthat hardness of the bearing surface is set to be equal to or higherthan hardness of the shaft so that frequency of replacement of thebearing due to wear life may be set to be lower than frequency ofreplacement of the shaft. In this case, hardness of the bearing surfaceneeds to be particularly increased.

In order to increase the hardness of the bearing surface of the sinteredbearing, a hardness increasing element (such as Ni, Mo, Mn, or Cr) maybe mixed into a material for sintered metal. However, theabove-mentioned hardness increasing element is expensive. Thus, when theentire sintered bearing is made of sintered metal containing thehardness increasing element, cost is increased. Further, an outerperipheral surface of the bearing serves as a mounting surface that ismounted to another member, and hence the outer peripheral surface needsto be finished into predetermined dimensions. However, when the entiresintered bearing is made of a material having high hardness, hardness ofthe outer peripheral surface is also increased, and hence processabilityof the outer peripheral surface is deteriorated. As a result, there is afear in that the outer peripheral surface cannot be finished withdesired dimension accuracy.

The above-mentioned circumstance is not limited to a case where thebearing surface is formed on an inner peripheral surface of the bearing.The same is true for a case where the bearing surface is formed on theouter peripheral surface. Note that, the “bearing surface” refers to asurface that slides so as to bear relative rotation between two members.The bearing surface may be formed on any one of the shaft side and thebearing side.

The present invention has an object to provide a sintered bearing havinga bearing surface increased in hardness, which can be manufactured atlow cost with high dimension accuracy.

Solution to Problems

According to one embodiment of the present invention, which is devisedto achieve the above-mentioned object, there is provided a sinteredbearing, comprising: an inner layer; and an outer layer formed on aradially outer side of the inner layer, the inner layer and the outerlayer being formed by integral molding, the sintered bearing having abearing surface formed on any one of an inner peripheral surface of theinner layer and an outer peripheral surface of the outer layer, whereinone of the inner layer and the outer layer, which has the bearingsurface, is made of sintered metal containing Fe and a hardnessincreasing element, and another of the inner layer and the outer layer,which has no bearing surface, is made of sintered metal containing Feand no hardness increasing element, and wherein a concentration gradientof the hardness increasing element is present at an interface betweenthe inner layer and the outer layer.

Note that, the hardness increasing element refers to an element thatcontributes to increase in surface hardness. For example, as thehardness increasing element, at least one kind selected from among Ni,Mo, Mn, and Cr may be used. Specifically, for example, in a case wherethe bearing surface is formed on the inner peripheral surface of theinner layer, the inner layer may be made of sintered metal containingFe, Cu, C, Ni, Mo, and inevitable impurities as the balance, and theouter layer may be made of sintered metal containing Fe, Cu, C, andinevitable impurities as the balance.

As described above, the layer having the bearing surface is made ofsintered metal containing the hardness increasing element, and thus itis possible to obtain the bearing surface increased in hardness.Further, the layer having no bearing surface is made of sintered metalcontaining no hardness increasing element, and thus an amount of usageof the hardness increasing element is reduced. Accordingly, it ispossible to reduce material cost. Further, the hardness increasingelement is not mixed into one of the layers (layer having no bearingsurface), and thus the one of the layers becomes softer than another oneof the layers (layer having the bearing surface) so as to be increasedin processability. Accordingly, it is possible to increase dimensionaccuracy of an outer peripheral surface of the sintered bearing.

Further, in the above-mentioned sintered bearing, the concentrationgradient of the hardness increasing element is present at the interface(boundary) between the inner layer and the outer layer. That is, in avicinity of the interface between the inner layer and the outer layer,there is formed a region containing the hardness increasing element andhaving high hardness. With this, the inner layer and the outer layer arefirmly bound together, and thus strength of the sintered bearing isincreased.

When Cu is mixed into at least one of the inner layer and the outerlayer, Cu functions as a binder to be able to firmly bind the innerlayer and the outer layer together. At this time, Cu contained in thelayer having the bearing surface contributes to increase in slidingproperty of the bearing surface, and hence a certain mixing amount of Cuis needed. However, it suffices that Cu in the layer having no bearingsurface function as a binder. Therefore, a mixing ratio of Cu in thelayer having no bearing surface may be lower than a mixing ratio of Cuin the layer having the bearing surface. As a result, an amount of usageof Cu is reduced, and material cost can be further reduced.

The above-mentioned sintered bearing can be suitably used at a jointsection of an arm of construction machinery.

Advantageous Effects of Invention

As described above, according to one embodiment of the presentinvention, it is possible to obtain the sintered bearing having abearing surface increased in hardness, which can be manufactured at lowcost with high dimension accuracy.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of a sintered bearing according to anembodiment of the present invention.

FIG. 2 is a plan view of the sintered bearing.

FIG. 3 is a graph showing a concentration gradient of a hardnessincreasing element.

FIG. 4 is a cross-sectional view illustrating a state in which a mixedmetal powder for an outer layer is filled in a compression-molding stepof manufacturing steps of the sintered bearing.

FIG. 5 is a cross-sectional view illustrating a state in which a mixedmetal powder for an inner layer is filled in the compression-moldingstep.

FIG. 6 is a cross-sectional view illustrating a state in which apartition plate is lowered in the compression-molding step.

FIG. 7 is a cross-sectional view illustrating a state in which a surplusof the metal powder is removed in the compression-molding step.

FIG. 8 is a cross-sectional view illustrating a state in which the mixedmetal powders are compressed by an upper punch in thecompression-molding step.

FIG. 9 is a cross-sectional view illustrating a state in which a greencompact is taken out of a die assembly in the compression-molding step.

FIG. 10 is a view illustrating the manufacturing steps performed afterthe compression-molding step.

FIG. 11 is a cross-sectional view of a sintered bearing according toanother embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

Now, embodiments of the present invention are described with referenceto the drawings.

As illustrated in FIG. 1 and FIG. 2, a sintered bearing 1 according toan embodiment of the present invention is made of cylindrical sinteredmetal, and is used at, for example, a joint section of an arm ofconstruction machinery. The sintered bearing 1 integrally comprises aninner layer 2 and an outer layer 3. In the illustrated example, thesintered bearing 1 is formed only of the inner layer 2 and the outerlayer 3, and both layers have a tubular shape, in particular, acylindrical shape. An inner peripheral surface of the sintered bearing 1is formed into a smooth cylindrical surface, and the sintered bearing 1has a bearing surface A for slidingly supporting a shaft 4 in arelatively rotatable manner. The shaft 4 is inserted along an innerperiphery of the sintered bearing 1. An outer peripheral surface of thesintered bearing 1 is formed into a smooth cylindrical surface, and thesintered bearing 1 has a mounting surface B that is mounted to anothermember. Each axial end surface of the sintered bearing 1 is also formedinto a flat surface. The sintered bearing 1 has, for example, an innerdiameter of 30 to 100 mm and a radial thickness of 5 to 50 mm. In thisembodiment, the sintered bearing 1 has an inner diameter of 35 mm and anouter diameter of 45 mm. A lubricant (such as oil and liquid grease) isimpregnated into inner pores of the sintered bearing 1, and the sinteredbearing 1 is held in slide-contact with the shaft 4. Thus, the lubricantseeps out from surface pores formed in the bearing surface A of thesintered bearing 1 so that the lubricant may be supplied to a slidingportion between the bearing surface A and the shaft 4.

The inner layer 2 is made of sintered metal containing Fe and a hardnessincreasing element. At least one kind selected from among, for example,Ni, Mo, Mn, and Cr may be used as the hardness increasing element. Theinner layer 2 according to this embodiment is made of sintered metalcontaining Fe, Cu, C, the hardness increasing elements (for example, Niand Mo), and inevitable impurities as the balance. Specifically, forexample, the inner layer 2 is made of sintered metal containing 15 to 20wt % of Cu, 0.3 to 0.8 wt % of C, 1.5 to 3.5 wt % of Ni, 0.5 to 1.5 wt %of Mo, and Fe and inevitable impurities as the balance. The bearingsurface A is formed on an inner peripheral surface 2 a of the innerlayer 2. In the illustrated example, the entire inner peripheral surface2 a of the inner layer 2 functions as the bearing surface A. A radialthickness of the inner layer 2 is set to approximately 5 to 20% of aradial thickness of the sintered bearing 1 (for example, set to 0.3 to 2mm). In this embodiment, the radial thickness of the inner layer 2 isset to approximately 1 mm. The reason is as follows. When the innerlayer 2 is extremely thin, a filling property of a raw-material powderat the time of molding is deteriorated and an allowable wear limit isreduced. When the inner layer 2 is extremely thick, an amount of usageof the hardness increasing element is increased, which leads to increasein cost.

As described above, the sintered metal of the inner layer 2 contains thehardness increasing element, and thus it is possible to obtain thebearing surface A that is increased in hardness and excellent in wearresistance. Further, the inner layer 2 contains Fe as a main componentand also contains C, and hence tensile strength and hardness can beincreased. Further, the inner layer 2 contains Cu, and hence the bearingsurface A is increased in sliding property, to thereby be able to reducefriction with the shaft 4. Further, as described above, when aquenching-property enhancing element such as Ni or Mo is selected as thehardness increasing element, it is possible to attain an effect oflowering a starting temperature of martensitic transformation.Accordingly, it is possible to achieve increase in hardness resultingfrom martensitic transformation in a cooling zone of a continuoussintering furnace in a sintering step described below.

The outer layer 3 is made of sintered metal containing Fe and nohardness increasing element (for example, none of Ni, Mo, Mn, and Cr).The outer layer 3 according to this embodiment is made of sintered metalcontaining Fe, Cu, C, and inevitable impurities as the balance.Specifically, for example, the outer layer 3 is made of sintered metalcontaining 2 to 5 wt % of Cu, 0.2 to 0.8 wt % of C, and Fe andinevitable impurities as the balance. The mounting surface B that ismounted to another member is formed on an outer peripheral surface 3 aof the outer layer 3. In the illustrated example, the entire outerperipheral surface 3 a of the outer layer 3 functions as the mountingsurface B.

As described above, the sintered metal of the outer layer 3 contains nohardness increasing element, and hence an amount of usage of theexpensive hardness increasing element is reduced. Thus, material costcan be reduced. Further, the sintered metal of the outer layer 3contains no hardness increasing element, and hence hardness of the outerlayer 3 can be lower than hardness of the inner layer 2. Accordingly,processability of the outer layer 3 is enhanced, and thus dimensionaccuracy of the mounting surface B can be increased.

Further, at least one (both in this embodiment) of the inner layer 2 andthe outer layer 3 contains Cu, and thus Cu functions as a binder throughmelting and binding. In this manner, a force of binding the inner layer2 and the outer layer 3 together is increased. In order to obtain thisfunction, it is preferred that the outer layer 3 contain 2 wt % or moreof Cu. Further, in order to reduce an amount of usage of Cu to achievereduction in cost, it is preferred that a mixing ratio of Cu in theouter layer 3 be lower than a mixing ratio of Cu in the inner layer 2.Specifically, it is preferred that the mixing ratio of Cu in the outerlayer 3 be set to 5 wt % or less.

A concentration gradient of the hardness increasing element is presentat an interface between the inner layer 2 and the outer layer 3. Theconcentration gradient is present over an entire axial region of theinterface between the inner layer 2 and the outer layer 3. In thisembodiment, as conceptually shown in FIG. 3, in a vicinity of theinterface (indicated by the dotted line) between the inner layer 2 andthe outer layer 3, specifically, in a radial region extending across theinterface, concentrations of Ni and Mo are gradually decreased from theinner layer 2 toward the outer layer 3. With this, a region containingthe hardness increasing element is formed in the vicinity of theinterface, and hence strength of the interface and also binding strengthbetween the inner layer 2 and the outer layer 3 are increased. It isdesired that a radial dimension R of a region (hereinafter referred toas a “concentration gradient region”), in which the concentrationgradient of the hardness increasing element is present, be set within arange of from 0.1 to 1.0 mm, preferably within a range of from 0.2 to0.5 mm. The reason is as follows. When the radial dimension R of theconcentration gradient region is extremely large, hardness of the outerlayer in the vicinity of the interface is increased, and hence there isa problem in that processability is deteriorated and dimension accuracyis adversely affected. When the radial dimension R of the concentrationgradient region is extremely small, binding at the interface isweakened, and hence there is a problem in that strength of the bearingis reduced. The radial dimension R of the concentration gradient regioncan be adjusted depending on, for example, a radial thickness of apartition plate 14 (see FIG. 5) of a two-color molding die assemblydescribed below.

The above-mentioned sintered bearing 1 is manufactured through, forexample, a compression-molding step, a sintering step, a reshaping step,a heat treatment step, and an oil-impregnating step. Now, each step isdescribed.

The compression-molding step is performed using, for example, a dieassembly illustrated in FIG. 4. The die assembly comprises: a die 11; acore pin 12 arranged along an inner periphery of the die 11; an outerlower punch 13 arranged between an inner peripheral surface 11 a of thedie 11 and an outer peripheral surface 12 a of the core pin 12; thepartition plate 14; an inner lower punch 15; and an upper punch 16 (seeFIG. 8). The outer lower punch 13, the partition plate 14, and the innerlower punch 15 have concentric cylindrical shapes, and can be raised andlowered independently of each other.

The compression-molding step is performed by so-called two-color moldingin which a material for the inner layer 2 and a material for the outerlayer 3 are fed into the same die assembly to integrally mold the innerlayer 2 and the outer layer 3. Specifically, first, as illustrated inFIG. 4, the partition plate 14 and the inner lower punch 15 are raisedto an upper end position, and the outer lower punch 13 is lowered to alower end position. Thus, the inner peripheral surface 11 a of the die11, an outer peripheral surface 14 a of the partition plate 14, and anend surface 13 a of the outer lower punch 13 form an outer cavity 17having a cylindrical shape. A first mixed metal powder M1 for formingthe outer layer 3 is filled into the outer cavity 17. The first mixedmetal powder M1 according to this embodiment contains a Fe powder, a Cupowder, and a C powder. Specifically, for example, SMF4030 (JISZ2550:2000) may be used as the first mixed metal powder M1.

Next, as illustrated in FIG. 5, the inner lower punch 15 is lowered tothe lower end position, and thus an inner peripheral surface 14 b of thepartition plate 14, the outer peripheral surface 12 a of the core pin12, and an end surface 15 a of the inner lower punch 15 form an innercavity 18 having a cylindrical shape. A second mixed metal powder M2 forforming the inner layer 2 is filled into the inner cavity 18. The secondmixed metal powder M2 according to this embodiment contains the Fepowder, the Cu powder, the C powder, a Ni powder, and a Mo powder. Atthis time, the second mixed metal powder M2 is caused to overflow theinner cavity 18 so as to cover an upper side of the partition plate 14.

Next, as illustrated in FIG. 6, the partition plate 14 is lowered. Thus,the second mixed metal powder M2 enters a space corresponding to thepartition plate 14, and the first mixed metal powder M1 and the secondmixed metal powder M2 are brought into contact with each other. In thismanner, a cavity 19 formed by the inner peripheral surface 11 a of thedie 11, the end surface 13 a of the outer lower punch 13, an end surface14 c of the partition plate 14, the end surface 15 a of the inner lowerpunch 15, and the outer peripheral surface 12 a of the core pin 12 isfilled with the first mixed metal powder M1 and the second mixed metalpowder M2 in a double-layer state. Then, a surplus of the second mixedmetal powder M2 overflowing the cavity 19 is removed (see FIG. 7).

After that, as illustrated in FIG. 8, the upper punch 16 is lowered, andthe mixed metal powders M1, M2 filled into the cavity 19 are compressedfrom above by an end surface 16 a of the upper punch 16. Thus, a greencompact M is molded. As illustrated in FIG. 9, the outer lower punch 13,the partition plate 14, and the inner lower punch 15 are raised, and thegreen compact M is taken out of the die assembly.

After that, in the sintering step, the green compact M is sintered at apredetermined sintering temperature (for example, 1,120° C.), and thus asintered compact M′ is obtained (see FIG. 10). In this embodiment, thesintering step is performed in the continuous sintering furnace. At thistime, the sintered compact M′ contains the quenching-property enhancingelements (Ni and Mo), and hence a starting temperature of martensitictransformation of the sintered compact M′ is lowered. Accordingly, it ispossible to achieve increase in hardness resulting from martensitictransformation of the sintered compact M′ in the cooling zone of thecontinuous sintering furnace.

The sintered compact M′ obtained through the sintering step is reshapedinto predetermined dimensions in the subsequent reshaping step. In thisembodiment, an inner peripheral surface, an outer peripheral surface,and both end surfaces of the sintered compact M′ are pressed by a sizingdie assembly, and thus the sintered compact M′ is die-molded intopredetermined dimensions (not shown). At this time, an outer layer M1′of the sintered compact M′ is made of relatively soft sintered metalcontaining no hardness increasing element, and has satisfactoryprocessability. Accordingly, the outer layer M1′, in particular, theouter peripheral surface of the sintered compact M′ can be molded withhigh accuracy.

Heat treatment is performed on the sintered compact M′ thus reshapedwith predetermined dimension accuracy (heat treatment step).Specifically, for example, tempering for eliminating internal stress ofthe sintered compact M′ is performed. Then, the lubricant is impregnatedinto the inner pores of the sintered compact M′ that has undergone heattreatment, and thus the sintered bearing 1 is completed.

The present invention is not limited to the above-mentioned embodiment.For example, in the above-mentioned embodiment, the reshaping step isperformed by die molding using the sizing die assembly, but the presentinvention is not limited thereto. The reshaping step may be performed byanother method such as machining. Further, in a case where desireddimension accuracy can be obtained without reshaping, the reshaping stepmay be omitted. Further, in the above-mentioned embodiment, tempering isperformed as the heat treatment step. However, quenching (for example,carburizing and quenching) may be performed before tempering, and thushardness of a surface of the sintered compact M′ may be increased. Notethat, when the inner layer 2 containing the hardness increasing elementhas adequate hardness, it is preferred that quenching be omitted toachieve reduction in cost.

Further, the above-mentioned embodiment exemplifies a case where thebearing surface A is formed on the inner peripheral surface 2 a of theinner layer 2, but the present invention is not limited thereto. Forexample, as illustrated in FIG. 11, the present invention may be appliedto the sintered bearing 1 in which the bearing surface A is formed onthe outer peripheral surface 3 a of the outer layer 3. In this case, theouter layer 3 is made of sintered metal containing Fe and the hardnessincreasing element, whereas the inner layer 2 is made of sintered metalcontaining Fe and no hardness increasing element. Specifically, forexample, the outer layer 3 is made of sintered metal containing Fe, Cu,C, Ni, Mo, and inevitable impurities as the balance, whereas the innerlayer 2 is made of sintered metal containing Fe, Cu, C, and inevitableimpurities as the balance. In this case, hardness of the outer layer 3is higher than hardness of the inner layer 2. The concentration gradientof the hardness increasing element is present at the interface betweenthe inner layer 2 and the outer layer 3, and in the vicinity of theinterface, concentration of the hardness increasing element is graduallydecreased from the outer layer 3 toward the inner layer 2. The mountingsurface B that is mounted to another member (outer peripheral surface ofthe shaft) is formed on the inner peripheral surface 2 a of the innerlayer 2. A thickness of the outer layer 3 is set to approximately 5 to20% of the radial thickness of the sintered bearing 1, that is, setwithin, for example, a range of from 0.3 to 2 mm.

Further, the above-mentioned embodiment exemplifies a case where theinterface between the inner layer 2 and the outer layer 3 assumes acylindrical surface shape, but the present invention is not limitedthereto. Across-section of the interface orthogonal to an axis can beformed into a non-circular shape (for example, polygonal shape or splineshape) (not shown). Thus, the binding strength between the inner layer 2and the outer layer 3 is further increased. At this time, the shape ofthe interface is parallel to an axial direction. The shape of theinterface is formed in conformity with a shape of the partition plate 14(see FIG. 5 and the like) in the compression-molding step, and hence theshape of the interface can be changed through change of the shape of thepartition plate 14.

Further, the above-mentioned embodiment exemplifies a case where thesintered bearing 1 is applied to construction machinery, but the presentinvention is not limited thereto. The present invention can be suitablyapplied to such an application that high surface pressure is applied tothe bearing surface.

REFERENCE SIGNS LIST

-   -   1 sintered bearing    -   2 inner layer    -   3 outer layer    -   A bearing surface    -   B mounting surface

The invention claimed is:
 1. A sintered bearing, comprising: an innerlayer; and an outer layer formed on a radially outer side of the innerlayer, the inner layer and the outer layer being formed by integralmolding, the sintered bearing having a bearing surface formed on any oneof an inner peripheral surface of the inner layer and an outerperipheral surface of the outer layer, wherein one of the inner layerand the outer layer, which has the bearing surface, is made of sinteredmetal containing Fe and a hardness increasing element, and another ofthe inner layer and the outer layer, which has no bearing surface, ismade of sintered metal containing Fe and no hardness increasing element,and wherein a concentration gradient of the hardness increasing elementis present at an interface between the inner layer and the outer layer.2. The sintered bearing according to claim 1, wherein the hardnessincreasing element comprises at least one kind selected from among Ni,Mo, Mn, and Cr.
 3. The sintered bearing according to claim 1, whereinthe bearing surface is formed on the inner peripheral surface of theinner layer.
 4. The sintered bearing according to claim 3, wherein theinner layer is made of sintered metal containing Fe, Cu, C, Ni, Mo, andinevitable impurities as the balance, and the outer layer is made ofsintered metal containing Fe, Cu, C, and inevitable impurities as thebalance.
 5. The sintered bearing according to claim 1, wherein at leastone of the inner layer and the outer layer contains Cu.
 6. The sinteredbearing according to claim 5, wherein a mixing ratio of the Cu in theanother of the inner layer and the outer layer, which has no bearingsurface, is lower than a mixing ratio of the Cu in the one of the innerlayer and the outer layer, which has the bearing surface.
 7. Thesintered bearing according to claim 1, wherein the sintered bearing isused at a joint section of an arm of construction machinery.